1. Field of the Invention
The present invention relates to mixtures of chromogens that are especially useful as color formers in carbonless copying systems.
2. Description of the Prior Art
Chromogenic mixtures that form "black" shades are highly desirable for use in pressure sensitive carbonless recording systems. "Black" images have superior reproduction characteristics when copied by xerographic processes. Additionally, "black" images provide excellent contrast, readability and are similar in appearance to traditional typewritten copy. In the context of carbonless systems, the term "black" refers to shades that range from dark gray to black in appearance and that are characterized by approximately straight line absorption throughout the entire visible range, approximately 400-700 millimicrons.
The traditional carbonless recording system includes a top sheet that is coated on its back surface ("CB") with a multitude of microcapsules containing a marking liquid and a bottom sheet coated on its front ("CF") with an acidic material, such as an acidic clay or a phenolic resin, that reacts with the normally colorless marking fluid upon rupture of the CB microcapsules to form an image on the CF. The marking fluid contained in the microcapsules coated on the CB is typically a mixture of chromogenic materials dissolved within a carrier oil or fluid.
Zinc-modified phenolic resins are now widely favored as the acidic material coated on the CF. This is due to their high reactivity, stabilizing effect on the formed images with respect to light and dark exposure and their low abrasiveness on paper coating equipment. However, zinc-modified phenolic resins display an unexpected inability to synergistically react with many mixtures of two or more chromogens. Rather, most blends of chromogens when imaged on zinc-modified phenolic resins show antagonism with respect to the imaging properties of each other resulting in undesirable shades, poor intensity, or both. This antagonism problem is particularly evident in chromogenic blends intended to form "black" images.
To date, the traditional solution to this problem has been the use of so-called "single component black" precursors. These chromogens are generally blackish green colored fluorans that are used alone or in combination with small amounts (5%-20% by weight) of toner chromogens in order to achieve a preferred "black" shade and to avoid the blending antagonism caused by zinc-modified phenolic resins. However, the use of "single component blacks" is undesirable from a commercial standpoint since they are generally quite expensive and must be applied in relatively large amounts. Thus, there is a need for a chromogenic mixture that will produce a "black" shaded image with zinc-modified phenolic resins while avoiding the antagonistic blending characteristics of such resins and at the same time eliminating or substantially reducing the amount of "single component black" chromogen used.
Most chromogenic mixtures include crystal violet lactone (3,3-bis(p-dimethylamino phenyl), 6-dimethyl amino phthalide) as one of the chromogenic components. For example, U.S. Pat. Nos. 4,376,150 (Morita et al.); 4,180,405 (Lawton); and 4,168,845 (Oeda et al.) all disclose chromogenic mixtures including, inter alia, CVL and a green chromogen. U.S. Pat. Nos. 4,363,664 (Delaney); 4,324,817 (Dahm et al.); 4,275,906 (Johnson et al.); 4,263,047 (Miyamoto et al.); 4,262,936 (Miyamoto); 4,197,346 (Stevens); 4,032,690 (Kohmura); 3,952,117 (Miyamoto); 3,940,275 (Brockett et al.); and 3,560,229 (Farnham et al.) all disclose chromogenic mixtures including, inter alia, CVL and various other fluoran homologs, isomers and analogs. These blends, however, suffer from antagonism problems when imaged on zinc-modified phenolic resins. In addition, the blends disclosed in the Brockett et al are blue, not black. U.S. Pat. Nos. 3,857,675 (Schwab et al.) and 3,849,164 (Schwab et al.) both teach blends of essentially green and red chromogens to produce a "black" shade that avoid the use of CVL entirely. See also U.S. Pat. No. 4,073,614 (Ozutsumi et al.).